libplacebo: add dependency to vulkan-headers also

libplacebo depends directly on vulkan-headers, not transitionally.
The constraint was written like this to not always install the headers
if some were provided by the system.

contrib/src/libplacebo/rules.mak

@@ -27,7 +27,7 @@ endif
 # We don't want vulkan on darwin for now
 ifndef HAVE_DARWIN_OS
 ifndef HAVE_EMSCRIPTEN
-DEPS_libplacebo += vulkan-loader $(DEPS_vulkan-loader)
+DEPS_libplacebo += vulkan-loader $(DEPS_vulkan-loader) vulkan-headers $(DEPS_vulkan-headers)
 endif
 endif